1. Field of the Invention
The present invention relates generally to media feed mechanisms, and more particularly to inhibiting media nip jump and rollback induced banding at the exit shaft.
2. Description of the Related Art
All-in-one machines typically perform functions such as printing, scanning, copying, and faxing in either a stand alone fashion or in conjunction with a personal computer and define a growing market for peripheral devices. These devices eliminate clutter in a business or home office by combining the desirable functionality of various machines into a single unit, while maintaining an affordable cost. Various all-in-one machines currently in the marketplace use thermal inkjet technology as a means for printing received fax documents, original documents, and copied or scanned images or text. Thermal inkjet printing devices utilize consumable inkjet cartridges in fluid communication with a printhead to record text and images on a print media. The printhead typically moves on a carriage relative to the media path and a control system activates the printhead to selectively eject ink droplets onto the print media.
The all-in-one devices utilize feed mechanisms configured to move sheets sequentially from the input tray, through a printing component and to an exit tray. Thus, feed mechanisms may include many parts which provide for media movement. Many feed mechanisms include drive transmissions which convert motor rotation into roller and shaft rotation to move media through the media path. The media is advanced in preselected steps or distances, also known as indexing, in order to properly form a printed image. Typically, these drive transmissions are gear drives, which require a necessary amount of tooth clearance, called backlash, for proper operation. Backlash is the amount of clearance between mated gear teeth in a gear pair. When media is passing through the printing component, any unintended advancement of media may result in print defects, such as banding or the like. Unfortunately, since proper gear design requires some backlash, unintended media movement is a continual problem. Some backlash is required to allow for lubrication, manufacturing errors, deflection under load and differential expansion between the gears and the housing. Backlash is created when the tooth thickness of either gear is less than the tooth thickness of an ideal gear, or the zero backlash tooth thickness. For example, standard practice is to make allowance for half the backlash in the tooth thickness of each pair.
During media feeding, at least two phenomenon may cause a printing defect known as banding. The first phenomenon that causes print banding is called media nip jump. When a media trailing edge exits a feed nip between a feed roll and the pressure or idler roll, the media is urged forward in a feed direction. This is due to the downward force of the biased idler roll stepping down from the media surface over the media trailing edge. Specifically, the downward force of the pressure roller causes a component force in the direction of media feed. The phenomenon is more pronounced when thicker media is utilized. Further, as the media disengages the feed system, the exit system becomes the sole driving force on the media. The exit system is typically overdriven, i.e. driven at a faster speed than the feed system, so that the media remains taut. This also causes media jump. The media may advance some undesirable distance corresponding to the backlash of a geartrain driving the feed roller. The result is that media may advance some distance greater than the intended amount.
The second phenomenon causing print defects is exit shaft rolling or rollback. Each time the motor rotates a preselected distance to index media through the feedpath, the motor stops. However, the exit shaft and rollers do not stop at the exact position and time that the motor stops at each indexing movement. This is due to several factors, such as the previously indicated backlash in the gear drive, commutator jump, exit system overdrive and other system tolerances. These tolerances are dampened to a large extent when the media is disposed within both the exit nip and feed nip because the feed system dampens the exit system overdrive and tolerances. However, when the media exits the feed system and is solely influenced by the exit system, the dampening effects of the feed system are lost and banding print defects are more visible to a user.
Given the foregoing, it will be appreciated that achieve benefits derived from overcoming the shortcomings and detriments described previously.